Addition effects of nanoscale NiO on microstructure and superconducting properties of MgB₂

pISSN 1229-3008 eISSN 2287-6251

Progress in Superconductivity and Cryogenics

Vol.18, No.1, (2016), pp.37~40 http://dx.doi.org/10.9714/psac.2016.18.1.037

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1. INTRODUCTION

The high critical current density (J

) at high fields and high temperatures is the prime requirement for MgB

to replace the low temperature superconductors, such as NbTi and Nb

Sn for cryogen-free practical applications [1-3].

Some of these applications include superconducting magnets for magnetic resonance imaging (MRI), solenoids, long power cables, and sensors for measuring liquid H

level [4, 5]. In these applications, pinning of magnetic flux lines (vortices) is an essential requirement to maintain high critical current at high fields for operation with low noise and lower power loss. Carbon doping is known to be very effective for enhancing the high-field critical current properties of MgB

[6]. However, the J

enhancement achieved at high fields are usually associated with J

degradation in low fields due to stronger current carrier scattering and current-blocking effects on grain boundaries [7]. At the same time, we have to compromise with the reduction of superconducting transition temperature (T

) in carbon doping. Therefore, we selected nanoscale NiO magnetic particles (mean particle size < 50 nm) as a dopant for MgB

to introduce magnetic impurities. Magnetic impurities usually have a stronger interaction with magnetic flux line than nonmagnetic impurities and may exert a stronger force to trap the flux lines if they can be properly introduced into the superconducting matrix [8]. In this work, we study the influence of NiO magnetic

nanoparticles on crystal structure, microstructure and superconducting properties of MgB

.

2. EXPERIMENTAL

NiO-added MgB

bulk samples were synthesized by the solid-state reaction method. The NiO with 0, 1, 3, and 5 wt. % of total MgB

was mixed with an appropriate amount of boron and magnesium powders under the environment of ultra-high pure Ar gas in glove box. The mixed powders were pelletized under 10 Ton pressure using mechanical hydraulic press. The pellets were put into Fe tubes and sintered in situ at temperature 800 °C for 30 min under the continuous flow of pure Ar gas.

The crystal structures of NiO-added MgB

bulk superconductors were investigated by X-ray diffraction (Rigaku, D/Max 2500) using Cu Kα as an X-ray source. In order to observe the microstructures, the samples were examined by scanning electron microscopy (SEM). The magnetization measurements were carried out on all samples by using a quantum design vibrating sample magnetometer option (Quantum Design PPMS). The superconducting transition temperature (T

) was obtained from zero-field-cooled (ZFC) measurement by first cooling the sample in zero field and then measuring the magnetic moment as the sample was warmed up in the field of 10 Oe.

To estimate the critical current density (J

) of samples, the magnetization hysteresis (M - H) loops were measured on all samples with magnetic field varying from -9 to 9 T applied parallel to the longest dimension of the sample.

Addition effects of nanoscale NiO on microstructure and superconducting properties of MgB <sub>2</sub>

Mahipal Ranot

, S. H. Jang

, Y. S. Oh

, K. P. Shinde

, S. H. Kang

, and K. C. Chung

a

Powder & Ceramics Division, Korea Institute of Materials Science, Changwon 642-831, Republic of Korea

b

Materials Deformation Department, Korea Institute of Materials Science, Changwon 642-831, Republic of Korea

(Received 11 February 2016; revised or reviewed 25 February 2016; accepted 26 February 2016)

Abstract

We have investigated the addition effect of NiO magnetic nanoparticles on crystal structure, microstructure as well as superconducting properties of MgB

. NiO-added MgB

samples were prepared by the solid-state reaction method. The superconducting transition temperature (T

) of 37.91 K was obtained for pure MgB

, and T

was found to decrease systematically on increasing the addition level of NiO. X-ray diffraction (XRD) analysis revealed that no substitution of Ni for Mg in the lattice of MgB

was occurred. The microstructural analysis shows that the pure MgB

sample consists of plate shape MgB

grains, and the grains get refined to smaller size with the addition of NiO nanoparticles. At 5 K, high values of critical current density (J

) were obtained for small amount NiO-added MgB

samples as compared to pure sample. The enhancement in J

could be attributed to the refinement of MgB

grains which leads to high density of grain boundaries in NiO-added MgB

samples.

Keywords : MgB

, nanoscale NiO, superconducting properties

* Corresponding author email: [email protected]

Addition effects of nanoscale NiO on microstructure and superconducting properties of MgB

3. RESULTS AND DISCUSSION

The X-ray diffraction (XRD) patterns for pure and different amounts of NiO-added MgB

samples are shown in Fig. 1. The MgB

diffraction peaks are clearly observed along with MgO peaks. MgO is really hard to avoid during synthesis process as Mg is a highly reactive material. No shift in MgB

peaks were observed for NiO-added samples (within the experimental error), this indicates that there are no changes in the a- or c-axis lattice parameters due to NiO addion. The unchanged lattice parameters suggest that there is no substitution of Ni for Mg was occurred.

However, for 5 wt. % NiO-added sample, MgNi compound was seen as a secondary phase in the XRD graph, which indicates Mg reacted with Ni upon increasing the level of NiO addion. No traces of free Ni or NiO particles were detected in the XRD patterns, this might be due to the small amount of NiO was used for addion. In order to clarify the presence of NiO nanoparticles in MgB

matrix, whether they are at grain boundaries or inside the grains, we are performing further experiments including microstructure observations and composition analysis by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), respectively.

Fig. 2 shows the temperature dependences of the magnetization for pure and different amounts of NiO-added MgB

samples. In ZFC curves all samples exhibit a sharp superconducting transition, which indicates that the samples are of good quality. The superconducting transition temperature (T

) of 37.91 K was obtained for pure MgB

. When we add NiO into MgB

, a systematic decrease in T

was found with increasing the addition level of NiO. For 1 wt. % NiO-added sample, T

decreased by 1.15 K, and it suppressed by 2.97 K for 5 wt. % NiO-added sample. Since Ni was not substituted for Mg in the lattice of

Fig. 1. X-ray diffraction patterns of pure and different amounts of NiO-added MgB

samples. No traces of free NiO particles were detected in doped samples.

Fig. 2. Temperature dependence of magnetization measured in applied field of 10 Oe for pure and NiO-added MgB

samples. A systematic decrease in T

was noticed with increasing the addition level of NiO.

MgB

, therefore, the reduction in T

because of pair breaking by magnetic NiO can be ruled out. On the other hand, the slow and systematic decrease of T

with NiO addition indicates that the drop in T

is most probably due to the change in stoichiometry of MgB

with varying the content of NiO and the reactions between Mg and Ni. The suppression of T

due to stoichiometry change has also been reported for dopants addition and their reactions with MgB

[9].

The high magnification SEM images of (a) pure and different amounts of NiO (b) 1 wt. %, (c) 3 wt. %, and (d) 5 wt. % added MgB

samples are presented in Fig. 3. The pure MgB

sample consists of plate shape crystalline grains with an average width of 120 nm and average length up to 425 nm. From SEM morphologies of NiO added samples, Figs. 3b to 3d, it appears that the grains are refined to smaller size with NiO addition. For 3 wt. % NiO-added sample, the average width of grains decreased from 120 nm of pure sample to 80 nm and the average length reduced from 425 nm to 400 nm. The grains refinement by NiO doping is also supported by the decrease in the full width at half maximum (FWHM) values (not shown here) for all diffraction peaks of MgB

.

To estimate the critical current density, the magnetization (M) as a function of applied field (H) at different temperatures was measured for all samples. As NiO is magnetic in nature, therefore, to check its magnetic effect, we measured the M - H loop at 40 K along with 5 and 20 K on a pellet sample of 3 wt. % NiO-added MgB

, and the data shown in Fig. 4. From M - H loop which was measured in the normal state (40 K), we can see that the signal from magnetic nanoparticles NiO-added MgB

sample is very negligible. This is most likely due to the small amount of NiO was added for doping purpose.

38

Mahipal Ranot, S. H. Jang, Y. S. Oh, K. P. Shinde, S. H. Kang, and K. C. Chung

Fig. 3. The SEM morphologies of (a) pure and different amounts of nano-NiO (b) 1 wt. %, (c) 3 wt. %, and (d) 5 wt. % added MgB

samples. The grains get refined to smaller size with NiO addition.

Thus, we can deduce the critical current density for all samples directly from M - H loops.

The J

was estimated from M–H loops by Bean’s critical state model, J

= 20ΔM/a(1-a/3b), where ΔM is the height of the M–H loop, a and b are the thickness and width of the sample, respectively. The magnetic field dependence of J

at 5 and 20 K for pure and NiO-added MgB

samples are shown in Fig. 5a and 5b, respectively. At 5 K, the small amount 1 wt. % and 3 wt. % NiO-added samples show higher J

both at low fields and up to 7 T as compared to pure sample. For further increasing the addition amount to 5 wt. %, the J

is similar to pure sample upto 5 T, but after that it starts to fall. This indicates that only small addition of NiO is effective to pin the vortices and maintain high J

. The high J

could be attributed to the refinement of MgB

grains by NiO addition, which increases the area of grain

Fig. 4. The magnetization hysteresis loops measured at different temperatures for 3 wt. % NiO-added MgB

sample.

Fig. 5. The critical current density as a function of magnetic field for pure and NiO-added MgB

samples measured at (a) 5 K and (b) 20 K. High J

values were obtained for small amount added NiO samples as compared to pure sample.

boundaries in MgB

. At 20 K, the J

of pure and NiO 1 wt. % and 3 wt. % added MgB

samples is almost overlapped with each other below 2 T field, after that the rate of decrease of J

increased upon increasing the addition level of NiO. More degradation in J

over the whole field was observed for 5 wt. % NiO-added sample.

This indicates that at high temperature (20 K) and at high fields the pinning efficiency of NiO magnetic nanoparticles is reduced. The size and the distribution of NiO magnetic nanoparticles in MgB

matrix and their flux-pinning mechanism will be our future research work.

4. CONCLUSION

In summary, the influence of NiO magnetic nanoparticles on crystal structure, microstructures as well as superconducting properties of MgB

were investigated.

The unchanged lattice parameters and no shift in the

diffraction peaks of MgB

indicate that there no

39

Addition effects of nanoscale NiO on microstructure and superconducting properties of MgB

substitution of Ni for Mg was occurred. A systematic decrease in T

was found upon increasing the level of NiO addition, which is most probably due to the change in stoichiometry of MgB

. It is observed that only small amount of NiO magnetic nanoparticles (≤ 3 wt. %) are effective in enhancement of J

, especially at low temperature of 5 K. The J

enhancement could be attributed to the refinement of MgB

grains by NiO addition, which increases the area of grain boundaries in MgB

, and the grain boundaries are known to be the dominant pinning source in MgB

superconductor.

ACKNOWLEDGMENT

This work was supported by Industrial Technology Innovation Program (Project number: 10053590) funded by the Ministry of Trade, Industry & Energy of Korea.

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